Esterification of amino-acids chelated to cobalt(III), molybdenocene, and platinum(II). Stability of chelated esters versus dechelation

Abstract

Several complexes of non-functionalized amino-acids, [Co(en)₂(GlyO)]²⁺, [Co(tren)(aa)]²⁺(en = ethylene-1,2-diamine, GlyO = glycinate), where aa = glycinate, alaninate, or leucinate anion, trans-[Pt(GlyO)₂], and [Mo(η-C₅H₅)₂(GlyO)]⁺ have been esterified either by alkylation with dimethyl sulphate or methyl toluene-p-sulphonate or acid-catalyzed esterification in methanol. With the cobalt(III) and molybdenocene complexes, a chelated aminoacid ester is obtained. The reactivity of the complexes is lower than that of the carboxylate salts in the alkylation reactions but comparable to that of the free acids in the acid-catalyzed reactions with methanol. With the cobalt and molybdenocene complexes, a slow dechelation reaction is observed in the presence of chloride ions to give the [Co(tren)(GlyOCH₃)Cl]²⁺(tren = 2,2′,2″-triaminotriethylamine) and [Mo(η-C₅H₅)₂(GlyOCH₃)Cl]⁺ ions. With the platinum complex, only a dechelated ester can be isolated.